Bromide as bleaching catalyst



ii'liittul 2,989,519 Patented June 20, 1961 2,989,519 BROMIDE AS 'BLEACHING CATALYST John H. E. H erbst, LOriginal, Ontario, Canada, and

Ilans A. Krassig, Hawkesbury, Ontario, Canada, asslgnors to Canadian International Paper Company, Montreal, Canada, a corporation of Quebec No Drawing. Filed Apr. 23, 1959, Ser. No. 808,298 Claims. (Cl. 260-212) This invention relates to reduction of the viscosity and degree of polymerization of cellulosic pulp in the alkaline hypochlorite bleaching process and has to do particularly with increasing the rate of the reduction.

In the manufacture of cellulosic materials, particularly in the manufacture of dissolving pulps for conversion to cellulose derivative products such as rayon, cellophane and the like, it is frequently necessary to reduce the viscosity, i.e., the degree of polymerization of the cellulose.

Reduction in the degree of polymerization means that the length of the cellulose molecules is reduced, resulting in lower viscosity of a cellulose solution produced by dissolving the fibers. Therefore, the viscosity, which may be measured by such methods as TAPPI Method T206m- 55 which involves solubilizing the cellulose fibers for test purposes, is an indication of the degree of polymerization of the cellulose.

The desired reduction in viscosity may be accomplished at any one or more of several stages in pulp refinement and conversion to a cellulose derivative and much of the reduction is generally accomplished in the alkaline hypochlorite bleaching stage. Nevertheless in its heretofore known form, the alkaline hypochlorite bleaching process has the disadvantage that a considerable amount of hypochlorite and an undesirably long time are required to attain the desired drop in viscosity, resulting in undesirably high cost. Another disadvantage is that pulp produced by the use of the usual hypochlorite process has relatively high solubility in boiling 7.14% sodium hydroxide solution and a relatively high copper number. These indicate a high content of carbonyl groups, an undesirable characteristic.

It is, accordingly, the object of the present invention to reduce the above disadvantages by providing a process which accomplishes reduction of viscosity in a substantially shorter time and with a subtsantial reduction in the amount of hypochlorite required to complete the reaction.

It is a further object to reduce the solubility in boiling 7.14% sodium hydroxide solution and to reduce the copper number, these being indications of undesirable characteristics.

Essentially, the invention comprises the addition of a small amount of bromine or a compound of bromine to the reaction mixture of the pulp and the alkaline hypochlorite bleaching solution in the range between 0.001 and 0.10% of bromine based on the pulp, i.e., ten to one thousand parts of bromine to a million parts of cellulose. The preferred compounds are salts of hydrobromic acid or hypobrornous acid. Other compounds of bromine may be used.

It is known that inhypochlorite bleaching the 7.14% hot sodium hydroxide solubility and the copper number of the pulp product can be reduced if the pH is increased. However, increase in pH reduces the rate of the reaction and has heretofore consequently been impractical because of time loss. A further advantage of the invention is that, because it increases the rate of the reaction, it is now practical to increase the pH, for instance, to 11.0 and to obtain even lower 7.14% hot sodium hydroxide solubilities and copper numbers than are obtained in the usual pH range, i.e., 9.5 to 10.5, with a saving in reaction time and consequent reduction in cost.

A further advantage of the invention is that since it accelerates the reaction in the lowering of viscosity, the reaction temperature may be reduced with a consequent saving in the cost of steam.

While we are aware that bromine has been prew'ously used in pulp bleaching, the ranges of amounts of bromine being used have been consistently much higher than the range of amount which we claim as our invention and the results of the prior use of bromine have been exactly opposite to the results which have been achieved by the present invention.

The primary object and the result of the present invention is to produce pulp of low viscosity. The following examples serve to illustrate the invention.

Example I The pulp used was a sulfite rayon pulp made from a wood mixture comprising 80% hardwoods (maples, elm, poplar) and 20% softwoods (spruce, balsam fir), which had been chlorinated and treated with a hot solution of sodium hydroxide. Its viscosity, determined according to TAPPI Method T206m-55, was 29 centipoises. Six parts of this pulp were bleached with 94 parts of an aqueous solution of sodium hypochlorite containing 2.4% of available chlorine (based on pulp) and sufficient sodium hydroxide to bring the pH at the end of the reaction to 9.510'.5. The temperature was 60 C. The time required to attain a viscosity of 9.7 centipoises, the 7.14% hot sodium hydroxide solubility, and the copper number according to TAPPI standard T215m-50 were determined. The brightness was measured with a General Electric recording spectrophotometer. The process was then repeated in the presence of 0.025 and 0.050% of sodium bromide, based on pulp; this corresponds to 0.019 and 0.039%, respectively, of bromide ion. The results are shown in the table.

Bromide ion added, percent 0 0.019 0. 039 Time to viscosity of 9.7 cp., hr.. 5. 4 4. 0 2. 0 7.14% hot NaOH soly., percent 12 11 10 Copper number 0. 9 0. 8 0. 8 Brightness, percent 94 94 94 Example 11 This was similar to Example I, except that the bleach addition was 1.8% of available chlorine, based on pulp, and that enough sodium bromide was added to give a bromide ion concentration of 0.078%, based on pulp. The time required to attain a viscosity of 9.7 centipoises was 1.7 hr., the 7.14% sodium hydroxide solubility 11%, the copper number 0.8, and the brightness 94%.

Example III Example IV Another lot of the type of pulp described in Example I was used; its viscosity was 32 centipoises. Except as shown otherwise, the bleaching conditions were the same as in Example I. Pertinent information is listed in the following table.

B1, Per- Av. 01, Ter- Bleach Av. Cl Brlght- 7.14%

cent on Percent mlnal, lug Consn., Vise, ness, NaOH Cu Pulp on Pulp pH Time, Percent cp. Percent Soly., N 0.

hr. on Pulp Percent Example V Two processes were performed, substantially in the manner described in the preceding examples. In the first, enough sodium bromide was added to give a concentration of 0.078% of bromide ion, while in the second, an equivalent amount of bromine was added. In the second, the addition of available chlorine was reduced by an amount equivalent to the amount of bromine added, and the addition of sodium hydroxide was increased by an amount equivalent to the amount of bromine added. The two processes gave substantially similar results.

Example VI To determine the effect of temperature reduction, a process performed at 60 C. without bromine was compared with two processes performed at 50 C. with sisting of elemental bromine, hydrobromic acid, hypobromous acid, and salts thereof in an amount between 0.001% and 0.10% based on the pulp.

2. The process as set forth in claim 1 in which said material is a salt of hydrobromic acid.

3. The process as set forth in claim 1 in which said material is a salt of hypobromous acid.

4. The process as set forth in claim 1 in which said material is elemental bromine.

5. Process of preparing oellulosic pulp for conversion to a cellulose derivative which comprises reducing the viscosity and degree of polymerizaion of the pulp fibers by treating the pulp with an alkaline hypochlorite bleach solution containing a material selected from the group bromine. Pertinent information is given in the followconsisting of elemental bfomim, hydfobfomic vadd, yp ing table. The pulp was the same as that used in the bromous acid, and salts thereof 1n an amount between processes of Example IV. 10 and 1000 parts per million parts of pulp.

Br, Per- Av. 01, Ter- Bleach- Av. 01 7.14% cent on Percent Temp., minal, lng Oonsn., Vise, NaOH Ou Pulp on Pulp 0. pH Time, Percent cp. Soly., No.

hr. on Pulp Percent 0 2. 1 10. 4 a. 4 1. s 9. 0 11. s 0. 0.078 2.1 50 10.7 2.8 1.1 9.0 11.1 0.81 0. 07s 1. 8 50 10. 7 3. 2 1. 2 9. 0 11. 2 0.79

We claim:

1. In the alkaline hypochlon'te bleaching of cellulosic pulp for conversion to a cellulose derivative, the process of increasing the rate of reduction of viscosity of the pulp which comprises adding to the reaction mixture of hypochlorite and pulp a material selected from the group con- References Cited in the file of this patent UNITED STATES PATENTS Callahan Apr. 18, 1933 Rapson Feb. 26, 1952 

1. IN THE ALKALINE HYPOCHLORITE BLEACHING OF CELLULOSIC PULP FOR CONVERSION TO A CELLULOSE DERIVATIVE, THE PROCESS OF INCREASING THE RATE OF REDUCTION OF VISCOSITY OF THE PULP WHICH COMPRISES ADDING TO THE REACTION MIXTURE OF HYPOCHLORITE AND PULP A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ELEMENTAL BROMINE, HYDROBROMIC ACID, HYPOBROMOUS ACID, AND SALTS THEREOF IN AN AMOUNT BETWEEN 0.001% AND 0.10% BASED ON THE PULP. 